The natural rubber (NR) used in the rubber industry is produced by solidification of the sap (latex) extracted from rubber trees called Hevea brasiliensis which are cultivated in tropical zones. Latex may be solidified, for example, by coagulating latex using acids such as formic acid and drying the coagula; or by allowing latex to naturally coagulate in latex collection cups at rubber plantations or coagulating latex with acids added to the cups to obtain cup lumps, and subjecting the cup lumps to repeated milling and washing, followed by drying and then pressing to produce natural rubber.
The natural rubber produced as described above contains, in addition to a polyisoprene component, many non-rubber components such as proteins, lipids, and saccharides. For this reason, these components can decay and emit offensive odors during the storage period prior to drying. Particularly in the case of cup lumps, the odor problem is more likely to occur because they contain a very large amount of non-rubber components, and they are stored for a long time at plantations and processing plants and during transportation and other stages. Nevertheless, due to their easy production and cost effectiveness, natural rubber produced from cup lumps has recently been widely used in tire applications. The odors of decayed natural rubber cause adverse effects, e.g. on the working environment and the surrounding environment not only of natural rubber processing plants but also of factories for manufacturing rubber products such as tires.
The following techniques for solving the odor problem of natural rubber have been proposed: a technique for reducing the odors by reacting natural rubber latex with proteases and surfactants to remove proteins which are one of the causes of decay (see, for example, Patent Literature 1); and a technique for reducing the rubber odors by adding inorganic salts and proteases to the serum separated during concentration and purification of natural rubber latex into natural rubber to produce natural rubber fine particles (see, for example, Patent Literature 2).
Moreover, Patent Literature 3 discloses a technique for reducing the odors by reducing the drying temperature in the production of natural rubber. Also proposed is a technique for reducing the odor components by immersing coagulated natural rubber latex in an alkali solution such as aqueous sodium hydroxide or aqueous potassium hydroxide (see, for example, Patent Literatures 4 and 5).